1. Field of the Invention
The invention relates to a multi-layer circuit board with a thickness of 1.6 mm, which can achieve impedance matching to result in reduced high speed signal reflection and reduced electromagnetic interference.
2. Description of the Related Art
FIG. 1 shows a conventional multi-layer circuit board 1 with a thickness of 1.6 mm. As shown, the conventional 1.6-mm multi-layer circuit board 1 includes: first, second, third, fourth and fifth insulating substrates (E1), (E2), (E3), (E4) and (E5) disposed sequentially one above the other; a first signal wiring layer (S1) disposed on one side of the first insulating substrate (E1) opposite to the second insulating substrate (E2); a ground wiring layer (GND) disposed between the first and second insulating substrates (E1), (E2); a second signal wiring layer (S2) disposed between the second and third insulating substrates (E2), (E3); a third signal wiring layer (S3) disposed between the third and fourth insulating substrates (E3), (E4); a power wiring layer (PWR) disposed between the fourth and fifth insulating substrates (E4), (E5); and a fourth signal wiring layer (S4) disposed on one side of the fifth insulating substrate (E5) opposite to the fourth insulating substrate (E4). The first, second, third, fourth and fifth insulating substrates (E1), (E2), (E3), (E4) and (ES), the first, second, third and fourth signal wiring layers (S1), (S2), (S3), (S4), the ground wiring layer (GND) and the power wiring layer (PWR) are press-bonded to each other to form the conventional circuit board 1 with a thickness of about 1.6 mm. The first and fourth wiring layers (S1), (S4) are adapted to be mounted with components 10 thereon. The third insulating substrate (E3) has a thickness of about 5.7 mil. Both of the second and fourth insulating substrates (E2), (E4) have a thickness of about 16 mil. Both of the first and fifth insulating substrates (E1), (E5) have a thickness of about 10 mil. Each of the first, third and fifth insulating substrates (E1), (E3), (E5) is made from a polyester prepreg. Each of the second and fourth insulating substrates (E2), (E4) is made from a fibrous core material that contains paper or glass fibers. In this structure, the first wiring layer (S1) has a first resistance (Rs1) with respect to the ground wiring layer (GND). The second wiring layer (S2) has a second resistance (Rs2) with respect to the ground wiring layer (GND) and the power wiring layer (PWR). The third wiring layer (S3) has a third resistance (Rs3) with respect to the ground wiring layer (GND) and the power wiring layer (PWR). The fourth wiring layer (S4) has a fourth resistance (Rs4) with respect to the power wiring layer (PWR). The first and fourth resistances (Rs1), (Rs4) are about 78 ohms. The second and third resistances (Rs2), (Rs3) are about 69 ohms. During layout, electrical connections pass through the wiring layers and the substrates. Such a conventional circuit board structure has certain drawbacks, which are set forth as follows:
(1) Serious reflection during high speed signal transmission: According to the standard theoretical values determined by Intel, the resistance between two adjacent wiring layers for a circuit board during high speed signal transmission is preferably within the range of 55.+-.10% .OMEGA., i.e., between 49.5 .OMEGA. and 60.5 .OMEGA.. However, the resistances of the conventional circuit board 1 fall outside the preferable range recommended by Intel. In addition, there is a difference of 9 ohms between the value of the first and fourth resistances (Rs1), (Rs4) and that of the second and third resistances (Rs2), (Rs3). Such a difference will result in an impedance mismatch. Thus, when a high speed signal is being transmitted through the conventional circuit board 1 and passes from the first or fourth wiring layer (S1) or (S4) to the second or third wiring layer (S2) or (S3), reflection of the signal will result. The reflection index of the high-speed signal can be calculated as ##EQU1##
When the reflection is serious, the waveform will be distorted considerably, thereby resulting in poor signal quality. In view of the aforesaid, the conventional 1.6-mm multi-layer circuit board 1 is not suited for high speed signal transmission.
(2) Weakened magnetic flux counteraction: As reflection of high speed signals will generate standing waves, which will increase electromagnetic radiation of the high speed signal, the magnetic flux counteraction of the circuit board 1 is weakened, thereby resulting in excessively high electromagnetic interference.